Industry is constantly searching for lighter, stronger, and more resistant materials to replace those in present use. Cyanate ester resins are known for their thermal stability, chemical inertness, solvent resistance, and dielectric properties. Thus, more and more uses are being found in a variety of fields which demand high performance materials, such as structural composites, printed wiring boards, semiconductor encapsulants, structural adhesives, graphic arts, injection molding and prepregs, and high performance binders.
Cyanate ester resins are formed from polyfunctional cyanate monomers (see U.S. Pat. No. 4,094,852). Generally, because it is desirable to achieve lower curing temperatures and faster curing times, a catalyst is employed. Catalysts which are effective include acids, bases, salts, nitrogen and phosphorus compounds, for example, Lewis acids such as AlCl.sub.3, BF.sub.3, FeCl.sub.3, TiCl.sub.4, ZnCl.sub.2, SnCl.sub.4 ; Bronsted acids such as HCl, H.sub.3 PO.sub.4 ; aromatic hydroxy compounds such as phenol, p-nitrophenol, pyrocatechol, dihydroxynaphthalene; various other compounds such as sodium hydroxide, sodium methoxide, sodium phenoxide, trimethylamine, triethylamine, tributylamine, diazabicyclo[2.2.2]octane, quinoline, isoquinoline, tetrahydroquinoline, tetraethylammonium chloride, pyridine-N-oxide, tributylphosphine, zinc octoate, tin octoate, zinc naphthenate, and mixtures thereof.
Oehmke (U.S. Pat. No. 3,694,410) teaches that chelates of metal ions of the nonionic type or ionic type, with 1 to 6 or more chelate rings, can catalyze the formation of polytriazines from aromatic polyfunctional cyanates. Similarly, Woo and Deller (U.S. Pat. No. 4,528,366) have shown that cobalt salts of C.sub.6-20 carboxylic acids are useful catalysts for polytriazine formation, preferably cobalt octoate and cobalt naphthenate. Shimp (U.S. Pat. Nos. 4,604,452 and 4,608,434) has shown that alcoholic solutions of metal carboxylates are effective catalyst compositions for polytriazine formation. Organometallic cobalt compounds have been used to catalyze the trimerization of acetylenes (U.S. Pat. No. 4,183,864) and the co-trimerization of acetylenes and nitriles (U.S. Pat. No. 4,328,343). The photocatalyzed cyclotrimerization of aryl isocyanates using metal carbonyl complexes has also been taught (E. Matteill, C. Pellizzi, and G. Predieri, J. Molec. Catalysis 22, 89-91 (1983)).
Energy polymerizable compositions comprising ionic salts of organometallic complex cations and cationically sensitive materials and the curing thereof has been taught [see European Patent Nos. 109,851, 1984; 094,914, 1983 and Derwent Abstract; and 094,915, 1983 and Derwent Abstract (English language equivalent So. Afr. 837966)]. EP 094,915 and EP 109,851 disclose curing in one and two stages. Energy polymerizable compositions comprising ionic salts of organometallic complex cations and polyurethane precursors or isocyanates has also been taught (U.S. Pat. No. 4,740,577 which also discloses curing in one and two stages; European Patent Nos. 265,373, 1988, Derwent Abstract; and 250,364, 1987, Derwent Abstract).
In certain applications, advantages in terms of improved potlife, physical properties of the cured material, and flexibility with respect to process parameters, particularly temperature, can be achieved with the use of photocatalysts. Gaku, Kimbura, and Yokoi (U.S. Pat. No. 4,554,346) disclosed (photo)curable resins from cyanate ester compounds. The inventors acknowledged that "the degree of radical-polymerizability or photo-polymerizability of the cyanate ester itself is small," resulting in poorly cured materials. Instead, Gaku, et al., used mixtures of polyfunctional cyanate esters with at least one compound having hydroxy group(s) and radical-polymerizable unsaturated double bonds, the compounds used in quantities such that the ratio of cyanato groups to the hydroxy groups is in the range from 1:0.1 to about 1:2, and a radical polymerization (photo)initiator, at elevated temperature. These materials would not be expected to yield the same polytriazine materials obtainable from direct polymerization of the polyfunctional cyanates to polytriazines. To our knowledge, no other photoinitiators or photocatalysts for polyfunctional cyanate curing to polytriazines have been disclosed.
What the prior art has not taught, but what the present invention teaches is the use of organometallic compounds for the curing of cyanate monomers.